1. Field of the Invention
The present invention relates to a die cushion apparatus of a press machine and to a surge pressure reduction method for a die cushion apparatus, more specifically to a die cushion apparatus of a press machine and to a surge pressure reduction method for a die cushion apparatus in which pressure oil is pressurized with the pressurizing force acting upon a die cushion pad and sealed gas is compressed by this pressure of the pressure oil.
2. Description of the Related Art
A die cushion apparatus has a function of pushing up the work that was fit into the lower mold, following the slide lift. However, the progress attained in the die technology resulted in that the blank holder function (a function of holding the peripheral portion of the work during deep drawing) is requested to be provided with the die cushion, and die cushions are now required to have a large capability, for example, of 300 tons or 400 tons.
In the usual die cushions, the lower surface side is supported by a pneumatic cylinder unit. This pneumatic cylinder unit is composed of a cylinder, a piston accommodated inside the cylinder and connected to the die cushion pad, and a pneumatic chamber. Air set to the prescribed pressure is sealed in the pneumatic chamber. During press operation, the pressure applied from the slide to the bolster acts upon the die cushion pad from the moment the upper mold and lower mold are brought into contact and to the moment the slide reaches the lower dead center. As a result, the die cushion pad and the piston of the pneumatic cylinder unit are moved down and the air sealed in the pneumatic chamber is compressed. Therefore, the movement of the die cushion pad is absorbed by the compression of air in the pneumatic chamber.
However, if the die cushion capacity is increased and the production rate further rises, an excessively large peak pressurizing force is generated at the instant of time the upper mold and lower mold are brought into contact. This peak pressurizing force is called a surge pressure, and a peak value increases in the air with a large compressibility. A variety of problems such as vibrations, noise, cracking of press frame, equipment failure, and low-quality processing of works were associated with such surge pressure. Accordingly, a technology has recently been used by which the die cushion pad was caused to move down immediately prior to the contact of the upper mold and lower mold in order to reduce the surge pressure. Such an operation control of the die cushion pad is called pre-acceleration. A Numerical Control (NC) die cushion shown, for example, in Japanese Examined Utility Model Application No. 7-47195 (column 4, lines 40–48, FIG. 1) is known as an example of the pre-acceleration.
FIG. 4 shows a hydraulic circuit of an NC die cushion. The lower surface of the die cushion pad 2 is supported by both a pneumatic cylinder unit 80 and the hydraulic cylinder unit 70. The hydraulic cylinder unit 70 is composed of a cylinder 71, a piston 72 sliding inside the cylinder 71, and first to third hydraulic chambers 73–75 separated by the piston 72. The piston 72 is connected to the die cushion pad 2 via a rod 76.
A pressure oil from the hydraulic pump is supplied to second and third hydraulic chambers 74, 75. This supply of pressure oil to the second and third hydraulic chambers 74, 75 is controlled by a servo valve unit 90. The servo valve unit 90 is controlled at a timing just before the contact of the upper mold and lower mold, and the pressure oil from the hydraulic pump 3 is supplied to the third hydraulic chamber 75. As a result, a pressure in the downward direction acts upon the piston 72, and the die cushion pad 2 moves down. For this reason, the relative speed of the upper mold and lower mold is decreased. The surge pressure is thus reduced.
In the NC die cushion shown in FIG. 4, supply of pressure oil to the first hydraulic chamber 73 and the discharge of the oil therefrom are controlled with the servo valve unit 94. Because the oil has small compressibility, if the pressure control of the first hydraulic chamber 73 is not conducted with good timing, the piston 72 to which the pressurizing force of the die cushion pad 2 is applied cannot operate and the apparatus is stopped frequently or, in the worst case, the apparatus is damaged.
The following control is also carried out in the NC die cushions.
If the die cushion pad 2 rises from the lower dead center as the slide passes the lower dead center, the work is sometimes deformed. Therefore, the die cushion pad 2 has to be temporarily stopped at the lower dead center. Such an operation control of the die cushion pad 2 is called locking.
In the NC die cushion shown in FIG. 4, the servo valve unit 90 is controlled and the circuit linking the second hydraulic chamber 74 and third hydraulic chamber 75 is closed at the timing when the die cushion pad has reached the lower dead center. As a result, the pressure oil located inside the third hydraulic chamber 75 is sealed. Therefore, even if the piston 72 tries to slide upward under the effect of pressure oil located inside the first hydraulic chamber 73, this operation is prevented by the pressure oil located in the third hydraulic chamber 75.
The servo valve used in the NC die cushion is easily affected by the impurities contained in the pressure oil. A small quantity of impurities mixed with the pressure oil prevent the servo valve from operating normally. Therefore, complicated operations such as maintenance of pressure oil have to be conducted frequently.
FIG. 4 shows a simplified servo valve unit, but actual servo valve units are composed of a large number of hydraulic mechanisms. Problems associated with such a configuration include a complicated piping system, increased cost, and a high malfunction frequency of the die cushion itself.
Furthermore, pressure control has to be carried out with good timing in order to conduct the pre-acceleration and locking effectively, and timing adjustment is a very difficult operation.